Superconductor with enhanced high magnetic field properties, manufacturing method thereof, and MRI apparatus comprising the same

ABSTRACT

A superconductor exemplarily described herein includes a superconducting material containing vortex pinning centers and non-magnetic disorders formed in the superconducting material. The superconductor described herein is suitable for use in magnet applications and power transmission.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of U.S. application Ser.No. 12/183,333, filed on Jul. 31, 2008, which claims priority from U.S.Provisional patent application Ser. No. 60/963,145 entitled “METHOD OFENHANCING HIGH MAGNETIC FIELD PROPERTIES OF SUPERCONDUCTING MATERIALS,WIRES, AND TAPES, INCLUDING Nb—Ti, Nb₃Sn, AND MgB₂” and filed on Aug. 1,2007, and U.S. Provisional patent application Ser. No. 60/964,441entitled “MgB₂ MAGNET-BASED MRI AND RELATED APPARATUS” and filed on Aug.10, 2007, the entire disclosures of which are incorporated herein byreference.

BACKGROUND

1. Field of Invention

Embodiments of the present invention relate generally to superconductorsand methods for manufacturing the same. More specifically, embodimentsof the present invention relate to a superconductor with high magneticfield properties, and to a method for enhancing high magnetic fieldproperties of a superconductor.

2. Description of the Related Art

Currently Nb—Ti alloy and Nb₃Sn are mostly used to produce high fieldmagnets. Recently discovered 39K superconductor MgB₂ has great potentialfor magnets, which has been intensively investigated (see C. Buzea andT. Yamashita, “Review of the Superconducting Properties of MgB₂”,Supercond. Sci. Technol. 15, R115-R146 (2001)). For power transmissionsuperconducting cables, made of the high T_(c) cuprates, are underdevelopment (see W. Buckel and R. Kleiner, “Superconductivity”,Wiley-VCH, Weinheim (2004), p. 382).

For magnet applications and power transmission, superconductingmaterials should have high critical current densities at high magneticfields, due to strong vortex pinning or flux pinning. Nb—Ti alloy andNb₃Sn show good pinning properties. Nevertheless, for special scientificapplications, such as accelerator applications, the high fieldproperties of these materials may need to be enhanced more. Furthermore,MgB₂, which can be used near 20K, does not exhibit good pinningproperties.

Pinning properties of superconducting materials can be enhanced byadding impurities, columnar defects, artificial pins, andnano-particles, mechanical alloying, introducing grain boundaries andprecipitates, and applying radiation damage (see W. Buckel and R.Kleiner, “Superconductivity”, Wiley-VCH, Weinheim (2004), p. 282).Changing the preparation conditions of samples also induces thedisorder, leading to the enhancement of high field properties. Thesemethods basically introduce the flux pinning centers intosuperconductors without reducing the critical temperature T_(c)significantly. Most progress has been made empirically, by trial anderror, because there is no reliable microscopic theory on the vortexpinning due to disorder, in general (see W. Buckel and R. Kleiner,“Superconductivity”, Wiley-VCH, Weinheim (2004), p. 284).

From a physical point of view, the above pinning centers essentiallyproduce the electric potential fluctuations, leading to the electrondensity fluctuations. Since the electron-phonon interaction, thederiving force of superconductivity, favors the electron densitycorrelations (see Mi-Ae Park and Yong-Jihn Kim, “Weak localizationeffect in superconductors from radiation damage”, Phys. Rev. B 61, 14733(2000)), these pinning sites may favor the flux penetration, destroyingsuperconductivity locally but allowing superconductivity overall, athigh magnetic fields in type II superconductors. In short, these knowntechniques employ electric potential fluctuations, i.e., electricproperties, to produce the pinning centers in superconductors. However,superconductivity near these pinning centers is destroyed due to theglobal energy minimization in the presence of flux penetration, ratherthan due to the local electric potential fluctuations. In other words,these potential fluctuations are not strong enough to destroy thesuperconductivity locally and therefore may not be the most desirablepinning centers.

In Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR)apparatus, the superconducting magnet forms a crucial part, because itdictates the operating temperature and the available magnetic fieldstrength. Currently, Nb—Ti and Nb₃Sn magnets are mostly used. However,the low transition temperature T_(c) of these materials requires liquidhelium, which leads to the high cost of the MRI and NMR apparatus andmaintenance. Therefore, it is highly desirable to find a higher T_(c)material for magnets of MRI and related NMR apparatus, which does notrequire expensive liquid helium.

In 2001, Akimitsu et al. found that MgB₂ is a superconductor withT_(c)=39K (see J. Nagamatsu et al., Nature, volume 410 (2001), p.63-64). Due to high T_(c), low cost, and good mechanical properties,MgB₂ has great potential for magnet applications. Indeed, in November2006, the first MgB₂ magnet-based MRI was introduced by ASGSuperconductors, Paramed Medical Systems, and Columbus Superconductors.The operating temperature of 20K was achieved using two cryocoolers,without using any cryogenic liquid. However, the MgB₂ magnet producedonly 0.5 T, limiting the marketability of the MgB₂ magnet-based MRI,because the current Nb—Ti and Nb₃Sn magnet-based MRI can produce 3 T at4K using the liquid helium. High magnetic fields provide better imagesbecause the resolution of the image depends on the square of thestrength of the magnetic field (see E. M. Haake, R. W. Brown, M. R.Thomson, and R. Venkatesan, “Magnetic Resonance Imaging: PhysicalPrinciples and Sequence Design”, (Wiley-Liss, New York, 1999), p. 6).

For magnet applications, superconducting materials should have highcritical current densities at high magnetic fields, due to strong vortexor flux pinning. However, MgB₂ does not show good pinning properties.Therefore, additives can be introduced into MgB₂ to enhance the vortexor flux pinning. For example, typical additives include: C (see S. X.Dou et al., Appl. Phys. Lett. 83, (2003) p. 4996), Al (see A. Berenov etal., Cond-mat/0405567 (2004)), SiC (S. X. Dou et al., Appl. Phys. Lett.81, (2002) p. 3419), Ti, Zr (see Y. Zhao et al., Appl. Phys. Lett. 79(2001) p. 1154), Si (see X. L. Wang et al., Physica C 385 (2003) p.461), Y₂O₃ (see J. Wang et al., Appl. Phys. Lett. 81 (2002) p. 2026),and Mg(B,O) precipitates (see Eom et al., Nature 411 (2001) p. 558).Although considerable progress has been made, more progress is requiredfor MgB₂ to be used for high field magnets.

SUMMARY

One embodiment exemplarily described herein can be generallycharacterized as a superconductor. The superconductor may comprisesuperconducting material containing vortex pinning centers andnon-magnetic disorders formed in the superconducting material.

Another embodiment exemplarily described herein can be generallycharacterized as a method for manufacturing a superconductor. The methodmay include preparing a superconducting material, forming vortex pinningcenters in the superconducting material and forming non-magneticdisorders in the superconducting material.

Yet another embodiment exemplarily described herein can be generallycharacterized as an MRI or an NMR apparatus comprising thesuperconductor described herein.

These and other aspects of the invention will become evident byreference to the following description of the invention, often referringto the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a superconducting wire according toone embodiment of the present invention, containing both magneticimpurities and non-magnetic disorders.

FIG. 2 is a schematic view illustrating the corresponding vortexconfiguration in a superconductor in FIG. 1.

FIG. 3 shows a schematic diagram of the temperature dependence of H_(c2)of La₃Al-based superconducting material, comparing properties aconventional example and an example according to one embodiment of thepresent invention.

FIG. 4 shows the critical current density J_(c) versus applied magneticfield at 5K for pure MgB₂ and 2% nano-Co₃O₄ doped MgB₂, comparingproperties of conventional examples and an example according to oneembodiment of the present invention.

FIG. 5 shows a schematic diagram of an MRI or NMR apparatus with a MgB₂magnet manufactured in accordance with embodiments of the presentinvention.

FIG. 6 is a schematic diagram of magnetic impurity concentrationdependence of J_(c) or H_(c2), illustrating effects of the magneticimpurities on the vortex pinning, and enhancement of high fieldproperties by the non-magnetic disorders according to embodiments of thepresent invention.

DETAILED DESCRIPTION

Embodiments of the present invention can be generally characterized ascapable of providing a method of enhancing the pinning properties and,therefore, high magnetic field properties of superconducting materialsfor magnet applications and power transmission. As described herein,pinning properties of superconducting materials may be enhanced usingthe properties of magnetic impurities to produce pinning centers insuperconductors, which destroy superconductivity locally. Non-magneticdisorders, such as impurities, defects, and the like, compensate(partially or completely) for the negative effects of magneticimpurities on the high field properties due to the significant decreaseof T_(c). Thus, adding non-magnetic disorders can amplify the high fieldproperties, i.e., vortex pinning, critical current density J_(c), andcritical magnetic field H_(c) (in particular, the upper criticalmagnetic field H_(c2)), because the corresponding vortex size is muchsmaller than that of a conventional vortex. Accordingly, overallsuperconductivity can be maintained while vortex pinning by magneticimpurities can be retained and amplified.

A recent scanning tunneling microscopy (STM) study of magnetic impurityeffect (see A. Yazdani, B. A. Jones, C. P. Luz, M. F. Crommie, and D. M.Eigler, “Probing the Local Effects of Magnetic Impurities onSuperconductivity”, Science Vol. 275, 1767 (1997)), can be understood toshow that superconductivity is destroyed within 1 nanometer of eachmagnetic impurity, implying the possibility of a vortex having a verysmall size. In that case, the upper critical field will be much higherand the vortex will be pinned strongly, because the flux through thevortex is quantized. In addition, Kim and Overhauser in other context,i.e., in the absence of magnetic fields (see Yong-Jihn Kim and A. W.Overhauser, “Magnetic Impurities in Superconductors: A Theory withDifferent Predictions”, Phys. Rev. B 49, 15799 (1994)) clarified thatT_(c) reduction induced by magnetic impurities can be compensated bynon-magnetic disorders in absence of a magnetic field. Nevertheless, itis unknown how to generalize the above-cited Kim and Overhauser theoryto solve vortex pinning due to the magnetic impurities.

Embodiments of the present invention are achieved by relying upon abetter understanding of the effects of magnetic impurities on vortexpinning, which was obtained from experimental data and realizing thelimitation of the above-cite Kim and Overhauser theory on this effect.First, the above STM data shows that magnetic impurities break Cooperpairs or destroy superconductivity near the magnetic impurities, leadingto the quasi-particle bound state with a nanometer length scale r_(o)(˜10 Å), not the much longer coherence length scale, ξ₀(ξ₀=˜50 Å forMgB₂ (see C. Buzea and T. Yamashita, “Review of the SuperconductingProperties of MgB₂”, Supercond. Sci. Technol. 15, (2001) p. R115-R146)).In other words, magnetic impurities are very strong pinning centers andthe corresponding the vortex size is about ˜10 Å, which increases theupper critical magnetic field H_(c2) significantly due to the fluxquantization.

In view of the above, magnetic impurities have both a positive effect onvortex pinning, by destroying superconductivity near impurities locally,and a negative effect on vortex pinning, by reducing the globalsuperconducting free energy and the corresponding superconductingtransition temperature T_(c). The negative effect increases when themagnetic impurity concentration is increases because the T_(c) reductioneventually leads to the reduction of the critical current J_(c) and theupper critical magnetic field H_(c2).

Forming non-magnetic disorders in the superconductor can suppress thenegative effect of magnetic impurities (partially or completely),therefore amplifying the positive effect of magnetic impurities. Forexample, the previous experimental study showed that only a very smallamount of magnetic impurities can enhance high field properties ofsuperconductors and that the enhancement is gone upon further adding asmall amount of magnetic impurities. This effect is shown in FIG. 6 byline 51. However, according to embodiments of the present invention,when non-magnetic disorders are added to the superconductor containingthe magnetic impurities, the critical current density J_(c) and/or theupper critical magnetic field H_(c2) will be significantly enhanced,which is highly non-trivial. This effect is shown in FIG. 6 at line 52.The theory by which high field properties can be enhanced bynon-magnetic disorders will be explained in greater detail withreference to FIGS. 1 and 2.

FIG. 1 illustrates a cross-sectional view of a superconductor inaccordance with an embodiment of the present invention.

The superconductor 10 may be provided as a wire. The superconductor 10may include superconducting material such as Nb—Ti alloy, Nb₃Sn, MgB₂and the like. Magnetic impurities 11 (depicted as arrows), may be formedin the superconducting material and function as vortex pinning centers.Non-magnetic disorders 12 (depicted as “x”s), are added to compensatefor the overall T_(c) decrease (partially or completely) due to magneticimpurities 11. In one embodiment, the spin of the magnetic impurities 11can be randomly oriented (i.e., paramagnetic), as shown in FIG. 1. Inother embodiments, the spin of the magnetic impurities 11 can beoriented ferromagnetic or anti-ferromagnetic in the superconductor 10.In one embodiment, the magnetic impurities can be provided as magneticnanoparticles, although the size of nanoparticles may be controlled toenhance the high field properties significantly.

FIG. 2 illustrates a schematic view of the corresponding vortexconfiguration in a superconductor in FIG. 1.

Referring to FIG. 2, lines 20 denote the magnetic field lines of force.The size of the vortex, ξ, is smaller than that of a conventionalvortex. In one embodiment, the size of the vortex, ξ, is of the order ofr_(o)=˜10 Å. The resulting vortex size and the upper critical magneticfield H_(c2) can be roughly estimated which, to the knowledge of theinventor, has never been accomplished before. The vortex size, ξ, willbe constrained by the size of the quasiparticle bound state, r_(o)=˜10Å, and the ratio of the mean free path (due to magnetic impurities 11),λ_(s), and the size of the Cooper pair (i.e., theBardeen-Cooper-Schrieffer (BCS) coherence length), ξ₀. When λ_(s) isalmost the same as ξ₀, the vortex size, ξ, will be comparable to thesize of the quasiparticle bound state, r_(o)=˜10 Å (more accurately,ξ≈2r_(o)=˜20 Å). However, when λ_(s)>ξ₀, the vortex size, ξ, will bebigger than 2r_(o) (i.e., comparable to ξ₀). Combining these two cases,the following is obtained:

$\begin{matrix}{\xi \approx {2\; r_{o}{{f\left( \frac{\ell_{s}}{\xi_{0}} \right)}.}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$

Here, “ƒ” means an appropriate function. For simplicity, “ƒ” may bechosen in the following way,

$\begin{matrix}{{\xi \approx {2\;{r_{o}\left( \frac{\ell_{s}}{\xi_{0}} \right)}^{a}}},} & {{Eq}.\mspace{14mu}(2)}\end{matrix}$with a certain exponent α. The corresponding Ginzburg-Landau coherencelength, ξ_(GL) is then given by

$\begin{matrix}{\xi_{GL} \approx {2\;{r_{o}\left( \frac{\ell_{s}}{\xi_{0}} \right)}^{a}{\left( \frac{T_{c}}{T_{c} - T} \right)^{\frac{1}{2}}.}}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$

Finally, the upper critical field, H_(c2) can be found as follows (seeW. Buckel and R. Kleiner, “Superconductivity” Wiley-VCH, Weinheim(2004), p. 236),

$\begin{matrix}{H_{c\; 2} = {\frac{\Phi_{0}}{2{\pi\xi}_{GL}^{2}} = {\frac{\Phi_{0}}{8\pi\; r_{0}^{2}}\left( {\xi_{0}/\ell_{s}} \right)^{2\; a}{\left( {1 - \frac{T}{T_{c}}} \right).}}}} & {{Eq}.\mspace{14mu}(4)}\end{matrix}$

In the presence of the non-magnetic disorders 12, such as impurities,defects, or the like, with the mean free path due to non-magneticdisorders λ<ξ₀, the Cooper pair size is reduced from ξ₀ to ξ_(eff)ξ_(eff)=√{square root over (λξ₀)}.  Eq. (5)

Consequently, ξ_(eff) is much bigger than 2r₀=˜20 Å in mostsuperconductors. Consequently, the vortex size is still constrained by2r₀ and Eq. (4) is still valid. It is stressed that magnetic impurities11 basically lead to the small size of the vortex and thereby lead tothe significant increase of the upper critical field H_(c2). Thecritical current density is also enhanced with almost the same rate,

$\begin{matrix}{{J_{c} = {{J_{c\; 0}\left( \frac{\xi_{0}}{r_{0}} \right)}^{2}\left( \frac{\xi_{0}}{\ell_{s}} \right)^{2\; a}}},} & {{Eq}.\mspace{14mu}(6)}\end{matrix}$if it is assumed that J_(c) is proportional to H_(c2) (see W. Buckel andR. Kleiner, “Superconductivity”, Wiley-VCH, Weinheim (2004), p. 271).Here J_(c0) is the critical current density for a pure superconductor.Accordingly, the irreversibility field, H_(irr), will be enhancedsignificantly.

Experimentally, it has been known that small amounts of magneticimpurities can enhance high field properties of superconductors. (see Y.Kuwasawa, K. Sekizawa, N. Usui, and K. Yasukochi, “Effects ofParamagnetic Impurities on Superconducting Properties in theLa_(3-x)Gd_(x)Al System”, J. Phys. Soc. Japan 27, 590 (1969); D. K.Finnemore, D. L. Johnson, J. E. Ostenson, F. H. Spedding, and B. J.Beaudry, “Superconductivity in Pure La and La—Gd”, Phys. Rev. 137, A 550(1965); and R. P. Guertin, J. E. Crow, A. R. Sweedler, and S. Foner,“Upper critical fields of superconducting Gd and Tm doped LaSn₃: Effectsof crystalline electric fields”, Sol. Sta. Commun. 13, 25 (1973)).However, such enhancement was obtained when only a small amount ofmagnetic impurities were provided and/or was accompanied by asignificant T_(c) decrease due to the magnetic impurities. Thus, suchenhancement is not practical.

Embodiments of the present invention provide a practical way tosignificantly enhance the high field properties of any superconductor,including Nb—Ti, Nb₃Sn, and MgB₂. For instance, FIG. 3 is a T_(c)-H_(c2)graph illustrating how embodiments of the present invention can beapplied to enhance the upper critical field, H_(c2), of La₃Al. It is hasbeen found that adding 0.3 at. % of Gd increases the slope of thetemperature dependence of H_(c2), while decreasing T_(c) about 1K. (seeY. Kuwasawa, K. Sekizawa, N. Usui, and K. Yasukochi, “Effects ofParamagnetic Impurities on Superconducting Properties in theLa_(3-x)Gd_(x)Al System”, J. Phys. Soc. Japan 27, 590 (1969)) Thisresult is shown at solid line 30. The dashed line 31 represents theexpected result when only non-magnetic disorders are added to La₃Al tocompensate for the T_(c) reduction, without taking into account theslope increase. Solid line 32 represents the result upon adding magneticimpurities and non-magnetic disorders to La₃Al. As shown, the slope ofthe solid line 32 is further increased compared to the slope of dashedline 31. A theoretical prediction, Eq. (5), quantifies the enhancementof H_(c2) due to both magnetic impurities and non-magnetic disorders,i.e., line 31 (in which λ_(s)=3.3494ξ₀, λ=0.5ξ₀, and α=1) and line 32(in which λ_(s)=2.817ξ₀, λ=0.5ξ₀, and α=1).

To the knowledge of the inventor, there has been no experimental studyfor the effect of magnetic impurity on vortex pinning in MgB₂. However,a few studies of the effect of the magnetic nano-particles on the vortexpinning in MgB₂ have been conducted. Magnetic nano-particles andmagnetic impurities will have almost the same effect on vortex pinning.FIG. 4 shows the critical current density J_(c) of MgB₂ at line 40, thecritical current density J_(c) of MgB₂ doped with 2% Co₃O₄nano-particles at line 41, and the critical current density J_(c) ofMgB₂ doped with 2% Co₃O₄ nano-particles as well as non-magneticdisorders in accordance with embodiments of the present invention atline 42.

Referring to FIG. 4, enhancement of the critical current density J_(c)of MgB₂ by adding 2% Co₃O₄ nano-particles has been reported by Awanda etal. (cond-mat/0601359). Awanda et al. found that J_(c) starts todecrease for 4% and 6% Co₃O₄ nano-particles, presumably due to the T_(e)decrease and the concomitant reduction of J_(c). Therefore, 2% dopingshows the maximum increase in critical current density J_(c) that hasbeen conventionally achieved. According to embodiments of the presentinvention, however, the critical current density J_(c) of MgB₂+2% Co₃O₄can be further enhanced by adding non-magnetic disorders. If enoughnon-magnetic disorders are added, with the mean free path, λ˜30 Å<ξ₀˜50Å, to reduce the Cooper pair size of MgB₂, the vortex pinning can beamplified while compensating for T_(c) reduction, leading to the furtheramplification of J_(c), as shown at line 42.

It has been found that samples with 0.13 wt. % Fe₂O₃ nano-particles showconsiderable enhancement of the magnetic hysteresis, whereas 0.26 wt. %added samples show moderate increase of the hysteresis. (see Prozorov etal. Appl. Phys. Lett. 83 (2003) p. 2019) Consequently, the enhancementof flux pinning due to the magnetic nano-particle doping is only limitedto the very low doping level, which is not that useful. On the otherhand, a decrease of critical currents due to the FeB nano-particles hasbeen found with the (higher) doping level above 1 wt. %. (see Dou etal., Supercond. Sci. Technol. 18 (2005) p. 710) It seems that Dou et al.might have found the enhancement for the lower doping level around 0.2wt. %. However, according to embodiments of the present invention,adding non-magnetic disorders into a superconducting material cancompensate for the negative effect (i.e., decrease of T_(c) or T_(c)) ofmagnetic impurities and at the same time amplify the positive effect(i.e., increase of H_(c2) and J_(c)). Thus, a superconducting transitiontemperature of superconducting material containing magnetic impuritiesand non-magnetic disorders can be greater than that of superconductingmaterial containing only the magnetic impurities and not thenon-magnetic disorders.

According to embodiments of the present invention, any availabletechnique can be used to introduce magnetic impurities and non-magneticdisorders into the superconductor. For example, techniques such asdiffusion, arc-melting, solid-state reactions, quenching-condensation,pulsed laser deposition (PLD), sputtering, molecular beam epitaxy (MBE),mechanical alloying, irradiation and implantation, chemical vapordeposition (CVD), powder-in-tube (PIT) technique, and the like may beused to introduce magnetic impurities and non-magnetic disorders intothe superconductor. The solubility of magnetic impurities within thesuperconducting material may be an issue. Therefore, magnetic impuritieswith the optimum solubility should be chosen depending on thesuperconductor material and the technique. If magnetic impurities areadded above the solubility limit, some magnetic impurities can formprecipitates. In this case, the precipitates still can contribute toincrease the critical current density J_(c), but the upper criticalfields H_(c2) will not increase much because the precipitates arebasically independent of the superconducting matrix. Adding magneticnanoparticles (see T. H. Alden and J. Livingston, “FerromagneticParticles in a Type-II superconductor”, J. Appl. Phys. 37, 3551 (1966);C. C. Koch and G. R. Love, “Superconductivity in Niobium containingferromagnetic Gadolinium or paramagnetic Yttrium dispersions”, J. Appl.Phys. 40, 3582 (1969); and A. Snezhko, T. Prozorov, and R. Prozorov,“Magnetic nanoparticles as efficient bulk pinning centers in type-IIsuperconductors”, Phys. Rev. B 71, 024527 (2005)), artificial magneticpins (see N. D. Rizzo, J. Q. Wang, D. E. Prober, L. R. Motowidlo, and B.A. Zeitlin, “Ferromagnetic artificial pinning centers in superconductingNb_(0.36)Ti_(0.64) wires”, Appl. Phys. Lett. 69, 2285 (1996)), andmagnetic dots (see G. Teniers, M. Lange, V. V. Moshchalkov, “Vortexdynamics in superconductors with a lattice of magnetic dots”, Physica C369, 268 (2002)), can lead to similar enhancement of critical currents,like the magnetic impurity addition. However, these techniques mayrequire fine tuning of nanoparticle size, which may not be adequate forthe mass production. Nevertheless, the negative effect of the magneticnanoparticles can also be compensated by adding non-magnetic disordersas discussed herein and the positive effect can thus be amplified by thenon-magnetic disorders. Accordingly, methods disclosed in embodiments ofthe present invention are advantageous because they can significantlyenhance the high field properties of superconductors, i.e., enhancementof both H_(c2) and J_(c) because it is intrinsic and powerful due to themagnetic impurity induced strong pinning of vortices.

According to some embodiments of the present invention, magneticimpurities can, for example, include at least one selected from thegroup consisting of an ion with partially-filled d-electrons (i.e., atransition metal) such as Mn, Fe, Ni, Cr, Co, Y, Zr, Nb, Mo, Tc, Ru, Rhand the like; an ion with partially-filled f-electrons (i.e., a rareearth element) such as Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Uand the like; and a magnetic particle (e.g., a magnetic precipitate).

According to some embodiments of the present invention, non-magneticdisorders can, for example, include at least one selected from the groupconsisting of a non-magnetic ion with an s-electron and/or a p-electronsuch as Zn, Al, Ti, C, B, Li, and the like. According to someembodiments of the present invention, non-magnetic disorders can, forexample, include any form of crystalline defect, such as at least oneselected from the group consisting of a vacancy defect, an interstitialdefect, a dislocation, radiation damage, and the like. According to someembodiments of the present invention, non-magnetic disorders can, forexample, include at least one selected from the group consisting of anon-magnetic particle (e.g., a non-magnetic precipitate), a second-phaseinclusion, a nano-sized particle, a segregate in a grain boundary of thesuperconducting material, and the like. It will also be appreciated thatthe non-magnetic disorders can include a combination of the aboveexamples. In one embodiment, the non-magnetic disorders can be formed inthe superconductor material by varying the processing conditions duringformation of the superconductor material. In another embodiment, thenon-magnetic disorders can be formed after the superconductor materialhas been formed. All these metallurgical problems should be taken intoaccount for the optimum combination of magnetic impurities andnon-magnetic disorders. For instance, combinations of Mn, Co, Fe, Ni,etc., as magnetic impurities and C, Zn, Al, etc., as non-magneticdisorders may be used in accordance with embodiments of the presentinvention.

MgB₂ has a rather small Cooper pair size in the range of 37-128Å (see C.Buzea and T. Yamashita, “Review of the Superconducting Properties ofMgB₂”, Supercond. Sci. Technol. 15, (2001) p. R115-R146). Although MgB₂does not have a weak link problem between the grain boundaries, theabundance of precipitates, nano-particles, or second phase inclusions,comparable to the size of the Cooper pair, in the grain boundaries maystill lead to reduced critical currents. Consequently, it is desirableto control the size of precipitates, nano-particles, and second phaseinclusions of the doping elements to be less than the size of the Cooperpair, which can then enhance the critical currents.

FIG. 5 shows a schematic diagram of an MRI or NMR apparatus with a MgB₂magnet manufactured in accordance with embodiments of the presentinvention.

Referring to FIG. 5, an MRI or NMR apparatus may include a MgB₂ magnet100 containing both magnetic impurities and non-magnetic disorders. TheMgB₂ magnet 100 may be capable of producing a magnetic field of 1˜3Tesla at 20K. Using the cryocooler 110, the MRI or NMR can be configuredto generate image for a patient lying in the scanner bore 120.

The optimum amount of magnetic impurities and non-magnetic disorders canbe determined from the optimum increase of the critical current densityJ_(c) and the upper critical magnetic field H_(c2) of thesuperconducting material, such as MgB₂. In one embodiment, theconcentration of magnetic impurities may range from 0.1% to 10% of thesuperconducting material, which corresponds to a 20˜30% decrease inT_(c), depending on the magnetic impurity and the superconductingmaterial.

In one embodiment, the concentration of non-magnetic disorders withinthe superconductor material can be estimated from the resulting meanfree path λ which should be comparable to, or smaller than, the BCScoherence length ξ₀ of the superconducting material such that:⅓≦ξ₀/λ≦3  Eq. (7)

In one embodiment, the concentration of non-magnetic disorders may rangefrom 1% to 20% of the superconducting material. In another embodiment,the concentration of non-magnetic disorders in the superconductingmaterial may be selected such that any non-magnetic disorder leads tothe mean free path λ and satisfies Eq. (7).

As will be appreciated, embodiments of the present invention may bepracticed in many ways. What follows in the paragraphs below is anon-limiting discussion of some embodiments of the present invention.

In one embodiment, a superconductor with a high magnetic field propertybelow a superconducting transition temperature includes: asuperconducting material having a superconductivity; magnetic impuritiesformed in the superconducting material; and non-magnetic disordersformed in the superconducting material. The non-magnetic disorders mayincrease an upper critical magnetic field or a critical current densityof the superconductor to a value greater than that in case of onlyforming the magnetic impurities in the superconducting material.

In another embodiment, a method for manufacturing a superconductor witha high magnetic field property below a superconducting transitiontemperature includes steps of: preparing a superconducting materialhaving a superconductivity; forming magnetic impurities in thesuperconducting material; and forming non-magnetic disorders in thesuperconducting material. The non-magnetic disorders may increase anupper critical magnetic field or a critical current density of thesuperconductor to a value greater than that in case of only forming themagnetic impurities in the superconducting material. The magneticimpurities and the non-magnetic disorders can be formed in a singleprocess during the preparation of the superconducting material, or afterformation of the superconducting material.

As exemplarily described herein, a superconductor contains both magneticimpurities and non-magnetic disorders (e.g., impurities, defects, andthe like), which enable high field properties of the superconductor tobe significantly enhanced. If magnetic impurities are added over thesolubility limit, they can form extra precipitates or nanoparticleswhich can still contribute to the enhancement of high field properties.Embodiments of the present invention can be easily adapted topowder-in-tube (PIT) MgB₂ wires and tapes. The resulting MgB₂ magnet isexpected to produce 1-3 T at 20K, which can replace the current Nb—Timagnet for MRI and related NMR apparatus. The magnetic impurity may bechosen to have an optimum solubility within the superconductingmaterial.

While embodiments of the present invention have been exemplarily shownand described above, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

1. A superconductor, comprising: a superconducting material containingmagnetic impurities; and non-magnetic disorders formed in thesuperconducting material.
 2. The superconductor of claim 1, wherein thesuperconducting material includes at least one selected from the groupconsisting of: Nb₃Sn, and MgB₂.
 3. The superconductor of claim 1,wherein the magnetic impurities function as to vortex pinning centers.4. The superconductor of claim 1, wherein the superconductor includesNb—Ti alloy comprising: Nb as the superconducting material; Ti as thenon-magnetic disorders; and the magnetic impurities.
 5. Thesuperconductor of claim 1, wherein the magnetic impurities include atleast one selected from the group consisting of: a magnetic ion withpartially-filled d-electrons, a magnetic ion with partially-filledf-electrons, and a magnetic particle.
 6. The superconductor of claim 1,wherein the magnetic impurities include at least one material selectedfrom the group consisting of: Mn, Fe, Ni, Cr, Co, Y, Zr, Nb, Mo, Tc, Ru,and Rh.
 7. The superconductor of claim 1, wherein the magneticimpurities include at least one material selected from the groupconsisting of: Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and U. 8.The superconductor of claim 1, wherein a concentration of magneticimpurities within the superconducting material ranges from 0.1 at. % to20 at. %.
 9. The superconductor of claim 1, wherein a superconductingtransition temperature of the superconducting material comprising themagnetic impurities and the non-magnetic disorders is greater than thatof the superconducting material comprising the magnetic impurities andnot comprising the non-magnetic disorders.
 10. The superconductor ofclaim 1, wherein the non-magnetic disorders include at least oneselected from the group consisting of a crystalline defect, anon-magnetic ion with an s-electron or p-electron, and a non-magneticparticle.
 11. The superconductor of claim 10, wherein the non-magneticdisorders include at least one crystalline defect selected from thegroup consisting of a vacancy defect, an interstitial defect, adislocation, and radiation damage.
 12. The superconductor of claim 10,wherein the non-magnetic disorders include at least one non-magneticparticle selected from the group consisting of: a precipitate, asecond-phase transition, nano-sized particles, and a segregate in agrain boundary of the superconducting material.
 13. The superconductorof claim 1, wherein the non-magnetic disorders include at least onematerial selected from the group consisting of Zn, Al, Ti, C, B and Li.14. The superconductor of claim 1, wherein a concentration of thenon-magnetic disorders within the superconducting material is selectedsuch that ξ₀/λ is in a range of 1/3≦ξ₀/λ≦3, where ξ₀ is a BCS coherencelength of the superconducting material and λ is a mean free path withinthe superconducting material comprising the non-magnetic disorders. 15.The superconductor of claim 1, wherein a concentration of thenon-magnetic disorders within the superconducting material ranges from 1at. % to 40 at. %.